Many computer graphic images are created by mathematically modeling the interaction of light with a three-dimensional (3D) scene from a given viewpoint. This process, called “rendering,” generates a two-dimensional (2D) image of the scene from the given viewpoint, and is analogous to taking a photograph of a real-world scene.
As the demand for computer graphics, and in particular for real-time computer graphics, has increased, computer systems with graphics processing subsystems adapted to accelerate the rendering process have become widespread. In these computer systems, the rendering process is divided between a computer's general purpose central processing unit (CPU) and the graphics processing subsystem, architecturally centered about a graphics processing unit (GPU). Typically, the CPU performs high-level operations, such as determining the position, motion, and collision of objects in a given scene. From these high-level operations, the CPU generates a set of rendering commands and data defining the desired rendered image or images. For example, rendering commands and data can define scene geometry, lighting, shading, texturing, motion, and/or camera parameters for a scene. The graphics processing subsystem creates one or more rendered images from the set of rendering commands and data.
Scene geometry is typically represented by geometric primitives, such as points, lines, polygons (for example, triangles and quadrilaterals), and curved surfaces, defined by one or more two- or three-dimensional vertices. Each vertex may have additional scalar or vector attributes used to determine qualities such as the color, transparency, lighting, shading, and animation of the vertex and its associated geometric primitives. Scene geometry may also be approximated by a depth texture representing view-space Z coordinates of opaque objects covering each pixel.
Many graphics processing subsystems are highly programmable through an application programming interface (API), enabling complicated lighting and shading algorithms, among other things, to be implemented. To exploit this programmability, applications can include one or more graphics processing subsystem programs, which are executed by the graphics processing subsystem in parallel with a main program executed by the CPU. Although not confined merely to implementing shading and lighting algorithms, these graphics processing subsystem programs are often referred to as “shading programs,” “programmable shaders,” or simply “shaders.”
Shadows are often rendered with respect to a single light source and then merged together with shadows rendered for each other light source. In computer graphics, scenes generally contain complex geometries and numerous light sources. Shadow rendering, or shading, per light source can become a critically long process in the graphics pipeline and is often afforded a great deal of attention to speed the processing.
Also of great importance is how realistic the rendered shadows turn out. Shadows experience a phenomenon known as contact-hardening. Contact-hardening occurs as a shadow-casting surface, or “blocker,” nears the surface being shaded, or the “receiver.” Shadows nearest the intersection are sharp or “hard,” and then soften or blur as the shadow extends away, creating a penumbra effect. Contact-hardening soft shadows are important for realistic rendering because the human visual system uses the variations of penumbra size as a cue for evaluating distances between shadow-casting and shadow-receiving objects.